It is known to provide an electrical device such as a fluorescent-lamp starter or a lamp, typically a halogen, with a twist- or turn-and-lock (TAL) base that comprises at least two pins that project parallel to each other from the device. Each pin has a small-diameter cylindrical shaft and a large-diameter cylindrical head and is made of metal, normally copper, for forming a good electrical contact to feed electricity to or through the device.
As described in German utility model 296 12 393 of E. Halemeier and German patent 1,589,233 of P. Rausseck the standard socket for such a device has a housing formed with a transverse wall having front and back faces. This wall is formed with a pair of throughgoing slots each having a wide end through which a respective one of the heads can pass and an opposite narrow end. Thus the device can be fitted to the housing by passing the heads of the pins through the wide ends followed by turning of the device to displace the heads toward the narrow ends of the respective slots which normally are formed as arcs with a common center of curvature. Respective electrical contacts on the housing at the slots engage the heads of the respective pins when same are at the narrow slot ends. Thus such a system is advantageous in that the device can be mounted or removed by a simple axial movement followed by or preceding an angular movement which is much simpler than screwing in the device.
Although the TAL base is of standardized size, with the pins set 7 mm apart for halogen lamps and 12.5 mm apart for starters, it is standard for the manufacturer of the socket to dimension the slotted end wall of the socket so that it has between its front and back faces a predetermined thickness substantially less than a distance between an end face of the device and the heads. This ensures that, even if the device is made to somewhat sloppy tolerances, it can be fitted to and locked in the socket. The normal tolerances for the contact pins is 0.2 mm and for the normally ceramic socket 0.3 mm. Thus the device is often a loose fit in the socket, something that is compounded when, as is common, the socket is made of ceramic material that shrinks somewhat over time when heated, often as much as 15%. The result is a loose fit that not only can leave a lamp off-center in its fixture, but that can actually come loose.
The terminals are often constituted with some spring action so as to radially retain the pins in place. While such spring-type terminals are often some help in retaining the device in the socket, they do not compensate out any poor fit caused by the difference between the spacing from the device end face to the head underside and the thickness of the slotted wall.